Preparation of single crystals of silicon



Sept. 9, 1958 s. E. BRADSHAW ETAL 2,851,342

PREPARATION OF SINGLE CRYSTALS 0F SILICON Filed Aug. 10, 1956 q'rroRueyg United States Patent "ice PREPARATION OF SINGLE CRYSTALS OF SILICON Stanley Edwin Bradshaw, Harrow Weald, and Abraham Isaac Mlavsky, London, England, assigncrs to The General Electric Company Limited, London, England Application August 10, 1956, Serial No. 603,327

Claims priority, application Great Britain August 25, 1955 1 Claim. (Cl. 23-301) This invention relates to the preparation of single crystals of silicon.

The invention is concerned in particular with methods of preparing single crystals of silicon of the kind in which a silicon seed crystal is dipped into a melt of silicon and is then progressively withdrawn upwards under conditions such that silicon from the melt progressively solidifies so as to form a single crystal propagated from the seed crystal.

According to the invention, a method of this kind comprises the preliminary step of maintaining the melt at a temperature at least 30 C. above the melting point of silicon under a vacuum corresponding to a pressure of not more than 10- millimetres of mercury, the vacuum being maintained thereafter during the growing of the single crystal.

Such a method results in a single crystal of relatively high purity, due to the evaporation of impurities from the melt; the time for which the melt is maintained at the elevated temperature is dependent upon the degree of purification required. Further advantages of the invention are that difliculties encountered in growing the single crystal due to the formation of a scum on the surface of the melt and difliculties encountered in the production of the melt due to violent agitation of the molten silicon are considerably lessened.

One method in accordance with the invention will now be described by way of example, with reference to the accompanying drawing, which is a side elevation, partly cut away to show internal details which are partly shown in section, of an apparatus for preparing single crystals of silicon.

Referring to the drawing, the apparatus includes a hermetically sealed enclosure constituted by a tubular metal member 1 to the ends of which are sealed top and base metal plates 2 and 3 respectively, the top plate 2 having sealed to it an inspection port 4 across the outer end of which is sealed a glass window (not visible in the drawing) through which operations carried out in the enclosure can be observed. The enclosure is connected to a pumping system (not shown) by a pumping tube 5 to which is connected an ionisation gauge 6.

Inside the enclosure is disposed a circular cylindrical crucible 7 of pure fused silica, the crucible 7 being seated within a circular cylindrical graphite cup 8 which is adapted to serve as an electric resistance heater. The cup 8 is formed integral with a downwardly extending circular cylindrical graphite skirt 9 which is split longitudinally so as to form two semi-cylindrical portions each of which has formed in it a slot into which fits one end of one of a pair of semi-cylindrical graphite members 10 and 11 which serve as supports and part of the lead system for the heater 8; the members 10 and 11 are themselves mounted on metal bars 12 and 13, which are in turn secured to metal bolts 14 and 15 which are sealed through the base plate 3 so as to be electrically insulated therefrom. The crucible 7 and heater 8 are surrounded by a heat reflecting metal baflle system 16,

2,851,342 Patented Sept. 9, 1958 and a further heat reflecting baffle 17 is disposed inside the skirt 9; the baffle system 16 and baflle 17 are supported by means of quartz rods such as 18 and 19 which are themselves mounted on a metal spider (not visible in the drawing) which extends across the opening of the pumping tube 5. In operation of the apparatus, the temperature of the heater 8 is measured by means of a noble metal thermocouple including elements 20 and 21, the hot junction of the thermocouple being disposed close to the base of the heater 8 and the cold junction of the thermocouple (not shown) being maintained in melting ice. Besides being utilised to operate an indicating instrument (not shown) the voltage generated by the thermocouple is fed to a control unit (not shown) which is adapted to maintain the temperature of the heater 8 substantially constant at any desired setting by automatic control of the power supply to the heater 8.

The apparatus also includes a holder for a silicon seed crystal which is in the form of a vertically extending rod 22 to the lower end of which is secured a chuck 23, the rod 22 passing through a gland 24 in the top plate 2 and being both vertically movable and rotatable about its longitudinal axis by means" of a suitable mechanism (not shown).

In carrying out the method in accordance with the invention, a quantity of about grams of solid silicon is placed in the crucible 7, the enclosure isevacuated by operating the pumping system so as to establish in the enclosure a vacuum corresponding to a pressure of the order of 10- to 10' millimetres of mercury as measured by the gauge 6, and the crucible 7 and its contents are then heated to a temperature of 30 C. above the melting point of silicon by energising the heater 8 so as to produce a pool of molten silicon 25. After about one hour a silicon seed crystal 26 mounted in the chuck 23 and having a horizontal cross-sectional area of 25 square millimetres is dipped into the molten silicon 25 by moving the rod 22 downwards, and the temperature of the molten silicon 25 is lowered to the point at which it begins to solidify on to the seed crystal 26. The rod 22 is then moved vertically upwards at a rate of 0.5 millimetre per minute so that silicon from the melt 25 progressively solidifies so as to form a single crystal propagated from the seed crystal 26, this single crystal being in the form of a vertically extending rod having a cross-sectional area of about seven square centimetres. In order to ensure homogeneous mixing, the rod 22 is rotated about a vertical axis at a speed of three revolutions per minute while it is being moved upwards. The process is terminated when substantially all the molten silicon 25 has been withdrawn from the crucible 7.

In order to illustrate the results obtained by use of the invention, the following case may be quoted. A silicon single crystal was grown from silicon which had been determined by spectroscopic analysis to contain as impurities, aluminium, antimony, boron, calcium, magnesium. nickel and zinc, and which had a resistivity of the order of 0.1 ohm centrimetre N-type. The resultant single crystal had a resistivity of the order of 10 ohm centimetres P-type, and analysis of material evaporated from the melt and deposited on cool parts of the enclosure indicated the presence of antimony, calcium, chromium, copper, germanium, lead, magnesium, nickel and zinc.

We claim:

In a method of preparing a single crystal of silicon in which a silicon seed crystal is dipped into a melt of silicon and is then progressively withdrawn upwards under conditions such that silicon from the melt progressively solidifies so as to form a single crystal propagated from the seed crystal: that improvement comprising the preliminary step of maintaining the melt at a temperature of at least 30 C. above the metling point of silicon under a vacuum corresponding to a pressure of not more than 10- millimetres of mercury and the subsequent step of maintaining the vacuum thereafter during the growing of the single crystal.

References Cited in the file of this patent UNITED STATES PATENTS 

